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<p><b>The Ionic Strength</b> corresponds to the concentration of electric charge
in a solution:</p>
<p class="indent">
<nobr><b><i>I</i></b> = &frac12; &sum; <i>m<sub>i</sub></i> <i>Z<sub>i</sub></i><sup>2</sup></nobr>
</p>

<p class="cont">
where <nobr><i>m<sub>i</sub></i></nobr> is the
<a href="SP_Molality.htm">molality</a> of an aqueous species <nobr>&#147;<i>i</i>&#148;</nobr>
with charge <nobr><i>Z<sub>i</sub></i>.</nobr>  For strong 1:1 electrolytes (like NaCl)
the ionic strength is equivalent to the total concentration.  The ionic strength is
used to calculate activity coefficients in dilute aqueous solutions.</p>

<p><a name="Act_Coef"></a>
<b>The activity coefficient</b>, <nobr>&#947;<sub><i>i</i></sub>,</nobr> gives the
relationship between concentration and thermodynamic activity:</p>
<p class="indent">
<nobr>&#947;<sub><i>i</i></sub> = <i>a<sub>i</sub></i> / <i>m<sub>i</sub></i> </nobr>
</p>
<p class="cont">
where <nobr><i>a<sub>i</sub></i></nobr> and and <nobr><i>m<sub>i</sub></i></nobr> are the
<a href="SP_Activity.htm">activity</a> and <a href="SP_Molality.htm">molality</a>,
respectively, of the aqueous species <nobr>&#147;<i>i</i>&#148;.</nobr> Note
that in equations concentrations are usually written with brackets,
for example <nobr>&#147;[H<sup>+</sup>]&#148;,</nobr> while activities are
often written with braces, <nobr>&#147;{H<sup>+</sup>}&#148;.</nobr></p>

<p><b>Activity coefficient calculations in <a href="S_0_Main.htm">SPANA</a>
(using <nobr><a href="SP_0_Main.htm">SED/PREDOM</a>):</b></nobr>

If the equilibrium constants in the input file are given for zero ionic strength
(standard conditions) then it is possible to give a value for the ionic strength,
<nobr><b><i>I</i></b>,</nobr> and the calculations will include the activity
coefficients calculated using the
the given values for <b>temperature</b> and ionic strength.</p>

<a name="Models"></a>
<h3>Models for activity coefficients</h3>

<p>In in <a href="S_0_Main.htm">SPANA</a> the user may choose a model to evaluate
activity coefficients if the <i>advanced</i> program level selected in the menu
&#147;Preferences / <nobr>General options&#148;.</nobr>
The following models may be selected:</p>

<ul>
<li><a href="SP_Ionic_Strength_Davies.htm"><b>Davies</b> equation</a></li>
<li><a href="SP_Ionic_Strength_HKF.htm"><b>simplified HKF</b> (Helgeson-Kirkham-Flowers) model</a></li>
<li><a href="SP_Ionic_Strength_SIT.htm"><b>SIT</b> (specific ion interaction theory)</a></li>
</ul>

<p>In the <i>normal</i> program level of <a href="S_0_Main.htm">SPANA</a>
activity coefficients are calculated using the
<a href="SP_Ionic_Strength_Davies.htm">simplified HKF model</a>.</p>

<a name="Comparison"></a>
<p><a href="SP_log_gamma.htm"><b>Comparison between the Davies <nobr>eqn.,</nobr>
the simplified HKF model, the SIT, and
some experimental data</b></a></p>

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